1. Field of the Invention
Aspects of the present invention relate to a power supply apparatus having multiple outputs, and more particularly, to a power supply apparatus having multiple outputs capable of individually controlling multiple output voltages using a simple structure.
2. Description of the Related Art
In general, apparatuses such as a computer, an image forming apparatus (such as a printer or a copier), a monitor, or a communication terminal, require an efficient power supply system that has a simple structure and a small size to provide a stable power supply. A current source type power supply apparatus has been widely used as an efficient power supply system.
FIG. 1 is a circuit diagram explaining basic operations of the current source type power supply apparatus. A circuit as illustrated in FIG. 1 is known as a flyback converter and is a type of DC/DC converter.
Referring to FIG. 1, the current source type power supply apparatus includes a transformer T having a predetermined turns ratio, a primary circuit 10 connected to a primary coil of the transformer T (i.e., connected to an input-side coil), and a secondary circuit 20 connected to a secondary coil of the transformer T (i.e., connected to an output-side coil). Here, the primary circuit 10 and the secondary circuit 20 can be insulated from each other by the transformer T.
The primary circuit 10 includes a control switch S connected in series between the primary coil of the transformer T and a grounding terminal. Here, the control switch S performs a switching operation on an input voltage in response to a control signal applied from an output voltage controller 30 in order to control an energy charging operation or a transferring operation of the transformer T.
The secondary circuit 20 includes a rectifier 21 to rectify currents transmitted from the transformer T. The rectifier 21 includes a diode D connected to the secondary coil of the transformer T and a capacitor C. Here, output terminals are provided at both end portions of the capacitor C. Therefore, an external load can be connected in parallel to both end portions of the capacitor C. In addition, although not shown in FIG. 1, the secondary circuit 20 may further include a filter to filter high frequency noise and electromagnetic interference (EMI) and an output voltage control circuit.
When the control switch S included in the primary circuit 10 is turned on, a voltage having an opposite polarity to that of the primary coil is induced in the secondary coil of the transformer T, so that the diode D of the rectifier 21 is in a reverse bias state. Therefore, a current flow to the secondary circuit 20 is blocked. Simultaneously, a magnetic inductance of the transformer T is charged with energy. More specifically, when the control switch S is in the “ON” state, current transfer by the transformer T does not occur and energy supplied to the primary coil is charged to the magnetic inductance of the transformer T.
On the other hand, when the control switch S is turned off, a voltage having an opposite polarity to a voltage in the “ON” state is induced in the secondary coil of the transformer T, so that the diode D of the secondary circuit 20 is in the “ON” state. Therefore, currents of the magnetic inductance with which the transformer T is charged are transmitted to the secondary circuit 20, and a DC voltage rectified by the rectifier 21 is output to the output terminal.
The output terminal of the secondary circuit 20 is connected to an output voltage controller 30. The output voltage controller 30 feeds an output voltage of the secondary circuit 20 back to the control switch S to apply a control signal to the control switch S. Here, the control signal is a signal to control a duty rate of the control switch S. Therefore, by controlling operations of the control switch S, the output voltage can be controlled.
As described above, when the control switch S included in the primary circuit 1 is turned on in the current source type power supply apparatus 20, magnetic inductance components of the transformer T are used as a boost inductor to charge the magnetic inductance of the transformer T. Conversely, magnetic inductance components of the transformer T are used to supply a DC output voltage that is rectified while currents of the magnetic inductance charged when the control switch S is turned off are transmitted to the secondary coil of the transformer T.
Therefore, the transformer T is used as a current source for the secondary circuit 20 that periodically supplies currents. As a result, the power supply apparatus utilizing such principles as shown in FIG. 1 is known as a current source type power supply apparatus. In addition to the aforementioned flyback converter, the current source type power supply apparatus may be of various types according to an additional circuit configuration of the primary circuit.
As compared with other types of power supply apparatuses, the rectifier of the secondary circuit of the current source type power supply apparatus has a simple structure and has a small number of components. Accordingly, the secondary circuit may be advantageous for the current source type power supply apparatus to use a multiple output structure. More specifically, for multiple outputs, a secondary circuit corresponding to each output has to be provided so that the simple structure of the secondary circuit may cause a decrease in the size of the entire apparatus. Due to this advantage, various types of current source type power supply apparatuses having multiple outputs have been introduced.
However, the conventional current source type power supply apparatuses having multiple outputs use multiple transformers, and include multiple regulator chips that may cause heavy losses in order to control a voltage output from each secondary circuit, or have complex structures in which an output voltage feedback circuit of each secondary circuit is connected to the primary circuit. As a result, the aforementioned advantage of the current source type power supply apparatus cannot be effectively applied.